Essential Genetics 6th Edition by Daniel Hartl

Exercise 16

Two classes of genes involved in segmentation of the Drosophila embryo are gap genes, which are expressed in one region of the developing embryo, and pair-rule genes, which are expressed in seven stripes. Larvae that are homozygous for recessive mutations in gap genes lack a continuous block of larval segments, and those that are homozygous for recessive mutations in pair-rule genes lack alternating segments. You examine gene expression by means of mRNA hybridization in situ and find that (1) the embryonic expression pattern of gap genes is normal in all pair-rule mutants, and (2) the pair-rule gene expression pattern is abnormal in all gap gene mutants.
What do these observations tell you about the temporal hierarchy of gap genes and pair-rule genes in the developmental pathway of segmentation?